Multi-site communication systems are well known. These communication systems provide communication services over a large area by dividing that area into subareas, known as cells, each being serviced by a discrete communication site. These systems require a handoff mechanism in order to provide seamless communication services to mobile communication units moving about the system coverage area. The handoff process switches the base site communication resource associated with a communication in progress from a first cell site to another cell site. Generally, this switch is enacted in response to degrading communication quality associated with the first communication site. This degradation in communication quality may be due to the mobile communication unit traveling too far from the first site, or to other factors affecting RF communication quality.
The general goal of any handoff method is to maximize the quality of service offered to the user at all times during a communication. There are three major factors that affect this quality. One is the inevitable interruption in the communication link as the user switches from the current communication channel to the new communication channel. Another is the signalling that controls the handoff. The final impact factor is the reliability of the new channel assignment; the handoff is of little use if the condition of the newly assigned communication channel is unacceptable, or acceptable for only a very short time.
Many methods for choosing a handoff channel are known. One typical handoff decision method requires the base stations at sites adjacent to the current site to monitor the signal quality of the mobile communication unit. Some methods produce a handoff that is substantially more reliable than other methods. Mobile-assisted handoff schemes are a good example of this, where the mobile communication unit is required to participate in the choice of handoff channels. The resulting handoff is generally more reliable because both the mobile communication unit and the base station are required to check the quality of the proposed handoff. Of course, this method requires the mobile communication unit to report its decision via additional signalling. Methods that act to improve the reliability of the handoff generally result in additional signalling and thus additional call interruption.
The application of handoffs to time division multiplexed (TDM) systems adds a new dimension, time, to the handoff problem. In a TDM system, the communication channel is defined as the combination of an RF frequency and a time slot. The duration of the call interruption for handoff and its requisite control signalling is directly related to the relative timing of the current communication channel (frequency plus time slot) and the new communication channel. Thus the expected call interruption will have a range of values dependent on the time slot corresponding to the chosen handoff channel.
A need exists for a method that better balances handoff methods with respect to these various factors, to attempt to maximize the quality of service to the user.